1. Field of the Invention
The present invention pertains to block copolymers of vinyl aromatic and conjugated diene monomers, and more particularly to coupling block copolymers together to form multi-block copolymers.
2. Description of the Related Art
The anionic polymerization technique to produce block copolymers is a well known process in the state of the art, and it is one of the more versatile techniques for the preparation of very well-defined block copolymers with narrow molecular weight distributions and with well-defined architectures and compositions. An interesting advantage of the anionic polymerization technique is the living nature of the polymer chains after consuming the added monomer. This advantage allows one to prepare sequentially perfect block copolymers, which retain the living nature at the end of a polymer chain. The living block polymer can be terminated by addition of an electrophilic compound or a proton donor compound such as carbon dioxide, oxygen, water, alcohols, mercaptans, and primary and secondary amines as described in U.S. Pat. No. 2,975,160. However, other types of terminating agents are coupling agents to form coupled copolymers having at least two branches in which they condense two or more living block copolymers into their molecule. For example, if two living linear block polymers are condensed into a coupling agent, the copolymer is a linear triblock copolymer, and if the final block copolymer has more than five arms or branches, it is called a radial block copolymer.
Radial block copolymers have many advantages over other types of copolymers in some specific applications, such as pressure-sensitive adhesives (PSA), engineering plastics, and polymer-modified asphalts (PMA). In pressure-sensitive adhesives, the use of a radial block copolymer can significantly increase the mechanical and adhesive properties of the PSA. Similar advantages are allowed if radial block copolymer is used to modify asphalt, because the mechanical and rheological properties of the asphalt are enhanced, and also the temperature susceptibility of PMA is reduced. Radial block copolymer can be used as a modifier for engineering plastics to improve the mechanical properties, such as impact resistance and tensile strength, of the plastics. However, if radial block copolymers are used, a large increase in the viscosity of the composite (i.e. PSA or PMA) has been observed in the past due to the effect of the number of arms, especially if it is greater than five arms. Therefore, viscosity is a key variable for processes where radial block copolymers are used to prepare adhesives, polymer-modified asphalts, and engineering plastic composites.
Bening et al. in U.S. Pat. No. 7,009,000, which is incorporated by reference, disclosed that by controlling the number of arms between 2 and 4 and decreasing the number of arms higher than 5 by using a metal alkyl compound, the viscosity of radial block copolymers can be held in an acceptable range. However, the resulting performance of PSA, PMA and engineering plastics is not good enough for certain applications.
Coupling efficiency depends on the type and amount of coupling agent, reaction medium (type of solvent), reactive terminal carbanion, and temperature. If coupling agent is added at an elevated temperature, terminating reactions compete with coupling reactions, and therefore, the coupling efficiency for a certain coupling agent is also decreased. Ambrose et al. in U.S. Pat. No. 4,075,186 disclosed that substituted polyacrylates can be used as a grafting agent to produce graft copolymers of polybutadiene and substituted polyacrylates with a coupling efficiency around 30-65%. Mancinelli in U.S. Pat. No. 4,503,188 disclosed that a multi-ester coupling agent can produce star block copolymers having a coupling efficiency of about 65%, according to examples therein. DuBois et al. in U.S. Pat. No. 5,397,841 disclosed that alkylmethacrylates can be used as coupling agents in anionic polymerization for producing grafted polymers, but coupling efficiency is around 80-90% for polystyrene, around 50-60% for polyisoprene and isoprene-styrene copolymers, and around 45% for styrene-butadiene copolymers. Deeter et al. in U.S. Patent Application Pub. No. 2005/0027071 A1, which was published Feb. 3, 2005, and which is incorporated by reference for all purposes, disclosed in their examples that coupling efficiency is no more than 60% because of the competition between terminating and coupling reactions when polyesters, polyacrylates, polymethacrylates or polyketones are used as coupling agents of styrene-butadiene copolymers in a solution anionic polymerization process.
Metal alkyl compounds may be used in an anionic polymerization process for several different applications. For example, Schade et al. in U.S. Pat. Nos. 6,300,441, 6,350,834, and 6,444,767; and Knoll et al. in U.S. Pat. No. 6,353,056 disclosed new initiator compositions using metal alkyl compounds as anionic polymerization retardant agents for producing anionic polymers. Willis et al., U.S. Pat. No. 6,391,981, disclosed that metal alkyl compounds are used to decrease the viscosity of the polymer cement. Desbois et al. in U.S. Pat. No. 6,686,423 disclosed that mixtures of metal alkyl compounds are better polymerization retardant agents than a single metal alkyl compound. Bening et al. in U.S. Pat. No. 7,009,000 disclosed that metal alkyl compounds increase the coupling efficiency when any di-ester compound is used as a coupling agent. Halasa et al. in U.S. Pat. No. 7,189,792, disclosed that organoaluminum compounds may be used as part of a new catalytic system to produce high 1,4-trans polybutadienes; and Willis in U.S. Pat. No. 7,125,940 disclosed that metal alkyl compounds can be used to control the microstructure of the polydiene part of block copolymers.
Polyesters, polyacrylates, polymethacrylates and polyketones compounds, when used as coupling agents, produce yellowness in the polymer cement and in the porous pellet product. Yellowness is an undesirable characteristic in many applications, such as in pressure-sensitive adhesives.